Other than repatriation or other out-of-the-ordinary circumstances: Once a vaccine has been widely applied or once treatments are available that sufficiently and consistently mitigate symptoms of COVID-19 infections.

It's not a matter of personal risk. It's one of trade-offs and risks at a societal level and from an economic point of view:

What could possibly be so important that it's worth risking getting an entire complement of passengers infected and possibly reintroducing the pathogen into a country for?